Spinel mixed

Table 5. Ligand-field parameters of spinel mixed crystals MgCrxAl2-x04, Cdo. Mgo.iCryAlz-yOi and ZnCr Ga -xOi (in cm t).

Spinel Mixed Metal Oxide Pigments. The crystal structure of spinel, MgAl204, is made up of an approximately cubic closely packed array of oxygen ions (Fig. 26). The unit cell contains 32 oxygen ions. Of the 64 possible tetrahedral sites, 8 are occupied by magnesium ions of the 32 possible octahedral sites, 16 are occupied by aluminum ions. 

Fig. 12 Absorption spectra of Fe O octahedra in spinel mixed crystals ZnGa2 xFex 04 top) and in the solid NH4Fe P207 (bottom)-, the best-fit band positions with respect to the igitzg Sg )...

Uzunova, E., Klissurski, D., Mitov, I. and Stefanov, P. (1993). Coball-iron hydroxide carbonate as a precursor for the synthesis of high-dispersity spinel mixed oxides. Chem. Mater., 5, 576-582. 

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

Manganates III), again mixed-metal oxides present in the spinel Mn304, Mn Mn 204. 

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

The structure of these solid compounds is not known with certainty but an approximate formula might be NaAlOj.xHjO. Many aluminates occur in minerals, for example the spinels of general formula M (A102)2 where M may be Mg, Zn or Fe these have a mixed oxide structure, i.e. consist essentially of M AF and O ions. 

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

Compounds that have the empirical formulas MCr02 and DCr204 where M is a monovalent and D a divalent cation, are known as chromites. These are actually mixed oxides and probably are better written as M20-Cr203 and D0-Cr203, respectively. The oxides of D are largely spinels, ie, the oxygen atoms define a close-packed cubic array having the octahedral holes occupied by the Cr(III) cation and the tetrahedral holes occupied by D (54). Chromite ore is an important member of this class of oxides. 

Results of quantum-mechanical calculations (Dunitz and Orgel, 1957) have given values for the OSPEs of a number of transitional metal ions and the degree of inversion of mixed spinels... 

Copper (Cu) and zinc (Zn), when mixed, form the alloy brass. Magnesia (MgO) and alumina (AljOj) when mixed in equal proportions form spinel. Iron (Fe) and carbon (C) mix to give carbon steel. 

Hammetter [91H01] has carried out DTA analysis of a shock-modified but unreacted 2A1 -t- Fe203 mixture and found a preinitiation event, indicating the presence of shock-produced Hercynite (spinel phase FeAl204). These preinitiation events observed in thermal analysis provide direct evidence for the influence of mechanical, shock-induced mixing in controlling solid state chemistry. 

This section considers a number of extremely important structure types in which A1 combines with one or more other metals to form a mixed oxide phase. The most significant of these from both a theoretical and an industrial viewpoint are spinel (MgAl204) and related compounds, Na- -alumina (NaAlnOi ) and related phases, and tricalcium aluminate (Ca3Al20g) which is a major constituent of Portland cement. Each of these compounds raises points of fundamental importance in solid-state chemistry and each possesses properties of crucial significance to... 

M2Ti04 (M = Mg, Zn, Mn, Fe, Co) have the spinel stmcture (MgAl204, p. 248) which is the third important stmcture type adopted by many mixed metal oxides in this the cations occupy both octahedral and tetrahedral sites in a cep array of oxide ions. Ba2Ti04, although having the same stoichiometry, is unique amongst titanates in that... 

Zeta process. This process improves the mixing and uses the -spinel LiAl508 as a spinel-former additive. In doing this the proportion of / "-alumina at 1260 °C is increased to 65 percent. 

Due to the formation of Ca/Al mixed oxide on the surface, the Ca -modified alumina has a completely different structure compared to the spinel one This leads to a different type of surface Lewis acid/basic sites, rendering the catalyst 30 times less active. 

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

---